Process for the preparation of magnesium by the electrolysis of its fused chloride



Pasma June. zo, 1939 PATENT OFFICE PBCESS FOR THE PREPARATION OF MAG- NESIUM BY T I IE ELCTBOLYSIS OF: ITS

FUSED CHLOEIDE chan-les de mmm, Neumy-sur-seine, France Application July 14, 1936, Serial No. 90,579

In Germany July 20, 1935 2 Claims.

When a mixture of fusedsalts containing magnesium chloride together with the chlorides of the alkali and alkaline earth metals is electrolysed, the electrolyte becomes .poorer in magnesium chloride and richer in. the other constituents. The electrolyte also becomes rlcher in impurities and particularly in MgO. This latter impurity is not only introduced together with the dehydrated magnesium chloride serving for the electrolysis, but is also formed during the electrolysis ifsmallquantities of water are present in or come into the electrolyte.

The preparation of pure anhydrous magnesium chloride is in general too complicated and costly for this substance to be employed' for the electrolytic preparation of magnesium, and heretofore it has been the. general practice to proceed by one of two well-known methods.

In the first of-these methods a mixture of chlorides is electrolysed until the magnesium chloride content of the bath has sunk so low as to make further electrolysis inadvisable. InV thev second method the zimpoverished electrolyte is withdrawn from time to time without interrupting the electrolysis, whereupon the electrolytic cell is filled with a previous charge'of the electrolyte so withdrawn whichhas subsequentlyNbeen enriched with magnesium chloride. If said first method is employed, it is necessary to start with an electrolyte containing a relatively high percentage' of magnesium chloride, as otherwise the labour involved inA charging and discharging i the cell becomes altogether disproportionate to the yield of metal. It is therefore impossible by this method to electrolyse with industrial success a mixture of salts containing less than 20% of magnesium chloride, which is the optimum value when current and energy efliclency are considered. The same considerations apply in a lesser degree to the said second method, which presents the added disadvantage that the solid impurities, and particularly the MgO, tend constantly to accumulate. Furthermore, both the.

aforesaid methods present the grave drawback that the level of the electrolyte in the cell varies with the progress of the electrolysis, thus causing variations in the anodic current density and accelerating the destruction of the anodes.

The object of the present invention consists in a process for the electrolytic preparation of magnesium from a bath containing fused magnesiumchloride. which process eliminates the disadvantages enumerated above and permits of the electrolysis being carried out under the optimum conditions.

. thus prolonging the life of the anodes.

' bath in which the magnesium chloride content (Cl. 204-l9) series of electrode systems act in succession upon an electrolyte in motion, fresh electrolyte being continuously added so as to maintain an approximately constant level in each one of the eleotrolytic cells. The impoverished electrolyte leaving the last electrode system is preferably m enrichedv with magnesium chloride and returned to the first electrode system. i

This method o f working permits of approachi ing the Optimum conditions for the electrolysis. The continuous and carefully regulated entry 15 and corresponding exit of the electrolyte maintain the level of the latter practically constant. Each system of electrodes continuously electrolyses a at any point is kept practically constant, and o the difference inconcentration between each succeeding el'ectrode system may, if so desired, be maintained slight, becoming less the greater the speed of circulation., This permits of obtainingand maintaining the optimum electrolytic conditions in each electrode system, and among others of maintaining the magnesium chloride content of the bath ineach electrode system at lessthan 30%, and preferentially at less than 20%.

v In order to put the invention into practice, it is `possible to employ either a series of cells, or an unique cell in which several series of electrode systems act one after the other upon theV electrolyte as. it traverses the cell. In both cases the-electrolytic cells are fed, either directly or indirectly, by means of a suitably heated container to which the impoverished electrolyteis returned, and where it is enriched in MgCl-z before being returned to'the cycle. This enriching of the impoverished electrolyte may be carried out in any known manner, but it is the' preferred practice to add, eithericontinuously or -discontinuously and in the solid or the fused state, a product consisting principally of hydrated MgClz and of a composition approaching the tetrahydrate, until the enriched electrolyte contains about 30% of anhydrous MgClz.

enriched and the electrolytic cells in which the masnesium is produced. The enriched electroi lyte is then allowed to decant in known manner i 'k -v w and settle out its solid impurities in this intermediary container. A plant for the electrolytic production of magnesium thus equipped can work efliciently for an indefinite period without being stopped.

The figure attached shows in diagrammatic Vertical section the principai means of execution whereby the invention may be put into effect, but it must be clearly understood that the means described are in no way to be construed as limiting the scope of the invention. a is a suitably heated container wherein the electrolyte is decanted from solid impurities in suspension." The eiectrolyte is then fed discontinuously to the heated container b from which it is admitted, by

means of a constant feed c to the flrst of a series of electrolytic celis d eachv having a cathode e and an anode The electrolyte'overfiows from cell to cell through the orifices h, being finally admitted to the suitably heated container o; to which MgCMHzO is alsoV fed at a constant rate so chosen as to ensure a bath having a composition approximating to 30% MgClz. The electrolyte'is then transferred discontinuously from o to the decantation cell a from whence it continues the previously described cycle.

It is moreover possible to put the invention into eiiicient practice even if the enriched electrolyte is not absolutely anhydrous. To this end the intermediary decantation container is suitably heated so as to eliminate 'the water present in the electrolyte to be fed to the electrolytic cells. Or again a slightly hydrous enriched electrolyte is submitted in known manner to va preliminary lelectrolysis under such working conditions of voltageand amperage that the water is decomposed without substantial decomposition of the magnesium chloride; When working. the processin this manner. it is advantageous to equip the oontainer in which the solid impurities are decanted with means whereby the water is electrolysed. V i

The inventloncan moreover be put into practice with pure magnesium chloride. or any bath contalning won-kable mixtures of magnesium chloride with the chlorides of more electropositive metals. Nevertheless the advantages of the present invention are most marked when electrolysing a bath containing less than 30% magnesium chloride.

I claim':

lf'A process of continuously producing magnesium by electrolysis of magnesium chloride in a fused electrolyte contalning magnesium chloride and a chloride of a more electro-positive metal than'niagnesium, and in which thev magnesium chloride is not in excess of 30% of the entire electrolyte. which comprises nowing said fused electrolyte in contact with a plurality of pairs of electrodes, while maintaining the level of the bath in contact with each particular pair of electrodes substantially constant, maintaining on each pair of electrodes a difference of potential sumcient to insure electrolysis of magnesium chloride with the production of magnesium, maintaining a continuous flow of said fused electrolyte in contact with said pairs of electrodes until the content of magnesium chloride therein has been substantially reduced but not eliminated. adding a quantity of hydrated magnesium chloride corresponding MSClz-HzO to said electrolyte, allowing sedimentation of magnesium' oxide from said mixture a'nd reintroducing the so-clarified and reinforced electrolyte to said treatment.

2. A process of continuously producing magnechloride is not in excess of 30% of the entire electrolyte, which comprises iiowing said fused electrolyte in contact with a plurality of pairs of electrodes, while maintaining the level of the bath in contact withl each particular pair of electrodes substantially constant, maintaining on each pair of electrodes a difference of potential sufflcient to insure electrolysisof magnesium chloride with the production of magnesium; maintaining a continous flow of said fused electrolyte in contact with said pairs of electrodes until .the content of magnesium chloride therein has been substantial- -ly reduced but not eliminated, adding a quantity of hydrated magnesium chloride to said electrolyte. allowing sedimentation of magnesium oxidefr msaid mixture and reintroducing the so-clari'- fled and reinforced eiectrolyte to saidtreatment.

CHARLES DI ROHDEN. 

